As electronic systems get smaller, faster and lower cost, the classic methods of separable interconnection need to be replaced with new technologies. One such technology is based on anisotropic conducting polymer materials. Anisotropic Conducting Elastomers (ACE) are elastomers which conduct in one direction but are insulators in the other direction. One such example is ECPI—(Elastomeric Conducting Polymer Interconnect) a material developed by Lucent Technologies—Bell Laboratories. This material is formed by magnetically aligning fine magnetic particles in sheets of uncured silicone such that the particles form arrays of electrically isolated columns. These columns are frozen in place as the silicone cures. When a layer of ECPI is compressed between two electrical conductors the particles in the compressed column come into contact with each other and the conductors, forming an electrically conductive path. Conductivity of the column remains over a compression range which is a function of the material design. This range, often referred to as the material's dynamic range, provides compensation for the lack of coplanarity of the conductors. This is often referred to as “coplanarity compensation”.
In a typical application of ACE the interconnect formed replaces the soldered interconnect to allow a separable interconnection. Separable interconnection is generally required for testing the device, conditioning the device (burn-in) and for final application in the OEM product. One such example is in a Land Grid Array (LGA) where an array of pads on a device needs to be connected to a matching array on a board. A second example is when a Ball Grid Array (BGA), consisting of a device with an array of solder balls, is to be separably connected to a matching array on the board. In both of these examples, a layer of ACE material placed between the device and the board can, when properly used, provide a reliable connection.
Further, when using ECPI as an interconnection medium between BGA devices, because the solder balls come in direct contact with the conducting particles on the outer surface of the ECPI, the spherical shape of the solder balls tend to bow the columns of particles outward from the contact center rather than compressing the columns in a straight line. The bowing may cause poor interconnection and shorting between adjacent pads.
Moreover, the behavior of the elastomeric material is critical to the success of the interconnect's performance. Typically highly filled elastomeric material exhibit poor elastic properties, and when formed into discrete button-like contacts, tended to move like putty, taking a severe set. These materials exhibit little residual spring force. These factors impact on the reliability of the contact, and virtually preclude multiple device insertions with different devices. Because these highly filled materials have poor elastic properties, an external spring member is required to create a contact force. However, the elastomer button-like contacts flow continuously under the force. The conventional solution is to limit the flow with a stop. The net effect is a very low contact force. In addition, elastomers in sheet form may have excellent elastic properties but tend to behave like incompressible fluids. This behavior demands that the connector system design provide for a place for the material to move.
Another problem arises because, unlike conventional pin-in-socket contacts which provide for a metal to metal wiping action as the pin mates with the socket, elastomeric contacts tend to provide no wipe. This wiping action breaks through surface contaminants and corrosion products, such as oxides and sulfides. Moreover, improper selection of contact materials, such as solder against a gold plated particle, can result in the gold dissolving in the solder and forming a brittle alloy which will break and form a type of insulating layer referred to as fretting corrosion.
The above described limitations to the capability of conventional elastomeric conducting materials may, to a varying extent, be applied to both anisotropically conducting elastomeric materials that have magnetically aligned particles, and isotropically conducting materials such as those that are heavily filled with conducting metal. These limitations also apply to some extent to anisotropic elastomeric materials that utilize other means than the magnetic alignment of particles to provide electrical and/or thermal connection.